1. Field of Invention
The present invention relates to an impeller and a centrifugal fan.
2. Description of the Related Art
In recent years, electronic parts arranged within an electronic device tend to generate an increased amount of heat as the electronic devices achieve advanced performance. A centrifugal fan is often used in cooling the interior of the electronic device. In this case, the centrifugal fan is required to have an increased air-blowing ability in keeping with the increase in the amount of heat generated, while also assuring noise reduction.
In order to increase the air-blowing ability of the centrifugal fan, there is a need to enhance the static pressure and flow rate characteristics thereof. The number of moving blades needs to be increased for the accomplishment of high static pressure but needs to be decreased for the accomplishment of high flow rate. In other words, the static pressure and the flow rate are in a trade-off relationship with each other. This poses a problem in that improvement in one characteristic results in deterioration in the other.
The moving blades severely interfere with the axially flowing air stream in the vicinity of an intake port of the centrifugal fan. Because of this, the increase in the number of moving blades entails a further problem in that the noise is increased by the interference of the moving blades with the air stream near the intake port.
Japanese Patent Laid-open Publication No. 2002-21782 discloses a conventional centrifugal fan in which the number of moving blades is greater in the radial outer side than in the radial inner side, thus assuring increased intake efficiency and improved flow rate characteristics (see, for example, paragraph [0029] and FIGS. 17 and 18 of Japanese Patent Laid-open Publication No. 2002-21782).
In the centrifugal fan disclosed in Japanese Patent Laid-open Publication No. 2002-21782, fan blades and wing portions are interconnected at the radial outer sides of an impeller by an annular ring plate extending along the circumferential direction about the center axis of the impeller. Thus, the annular ring plate is apt to become an obstacle against the air stream generated by rotation of the impeller. This may result in a deteriorated flow rate characteristic, increased noise, and other problems.